1. Field of the Invention
The present invention relates in general to a rotating motor machine in which a load element is driven by a motor element, and in particular to a torque control apparatus suitable for reducing the vibration of a rotating motor machine.
2. Description of the Prior Art
As an example of a rotating machine driven by a motor element, an enclosed rotary compressor will now be described. FIG. 16 is a longitudinal section view showing the conventional structure of an enclosed rotary compressor. FIG. 17 is a cross sectional view seen along a line XVII-XVII of FIG. 16. In FIGS. 16 and 17, numeral 1 denotes a compressor case housing a motor element and a compression element therein. Numeral 2 denotes a stator of the motor element fixed to the internal circumference of the case 1. Inside the stator 2, a rotor 3 of the motor element fitted to a main shaft 4 so as to rotate together therewith is disposed. The main shaft 4 is supported by main bearing and terminal bearing 6. The bearing 5 and 6 are coupled to a cylinder block 7. The cylinder block 7 is fixed to the case 1. The cylinder block 7 is fixed to the case 1. A compression space 12 is formed within the cylinder block 7. In the compression space 12, a roller 8 which is integral with the main shaft 4 is so disposed as to be eccentric. A vane 9 is so disposed as to be pressed against to the surface of the roller 8 by spring 10. These constitute the compression element. When the main shaft is driven to rotate by the above described motor element under such configuration, coolant gas is sucked from a suction accumulator 11 disposed outside the case 1, pressurized to a predetermined pressure in the compression space 12 by the roller 8, and discharged out of the case along the direction indicated by arrows. A rotary compressor having such a structure is disclosed in JP-A-58-187635.
In a compresser having such a structure, the gas suction torque in the compression element changes greatly during one revolution of the main shaft 4, whereas the motor element outputs a nearly constant torque with respect to time. The difference between the gas suction torque and the electromagnetic torque, i.e., the residual torque serves as an exciting torque with respect to the case 1. In the compressor as a whole, large vibration is induced in the rotation direction, resulting in a problem.
FIG. 18 schematically shows a cause of vibration generation in a rotary compressor as an example. With respect to a rotating side (the rotor 3, the roller 8 and the main shaft 4) and a stationary side (the stator 2, the block 7 and the container 1) of the compression element and the motor element, a gas compression torque T.sub.G and the electromagnetic torque T.sub.M function as illustrated in FIG. 18. In FIG. 18, the clockwise direction is represented by the plus sign and the counterclockwise direction is represented by the negative sign. At this time, equations of motion of the rotating side and the stationary side are represented as ##EQU1## are respectively inertia torques of the rotating side and the stationary side, J.sub.R and J.sub.S are respectively inertia moments of the rotating side and the stationary side, T.sub.G -T.sub.M is equivalent to the residual torque (=excitation torque), K is a spring constant of the spring for supporting the compressor, and .phi..sub.R is a rotation angle of the rotating side, which is related to the rotational angular speed w.sub.R of the rotating side as ##EQU2##
If the rotational speed w.sub.R of the rotating side is detected and feedback control is so performed that the rotational speed w.sub.R may always be equal to a constant commanded value of the rotational speed, therefore, it follows that ##EQU3## From the equation (1), therefore, the relation T.sub.M -T.sub.G =0 is satisfied. Since the electromagnetic torque T.sub.M thus balances the load torque T.sub.G, the excitation torque in the equation (2) is eliminated. As a result, it becomes possible to suppress the vibration of the case in the rotation direction which might cause vibration and noise of the rotating machine.
As described in JP-A-62-83530, the load torque T.sub.G of the compressor changes very largely during one revolution. However, the load is a periodically pulsating load repeated with a period of one revolution. If attention is paid to a particular rotation angle, the load torque value at that rotation angle does not significantly change. Paying attention to this fact, the rotational speed detected at each rotation angle is fed back at the same angle appearing after one revolution to perform the torque control in an example of the prior art described in JP-A-62-83530.
In general, however, the variation curve of the rotational speed and the variation curve of the load torque T.sub.G differ from each other in phase. FIG. 19 shows a load torque curve of a rotary compressor together with a rotational speed variation curve of a rotor. As evident from FIG. 19, the rotational speed varies with a lag phase with respect to the torque variation. The point where the rotational speed becomes the minimum is several ten degrees behind the peak value of the load torque.
In the above described torque control comprising the steps of detecting the rotational speed of a rotor, increasing the motor output when the detected speed is smaller than the commanded speed, and decreasing the motor output when the detected speed is larger than the commanded speed, therefore, it is not possible to completely synchronize the motor output torque to the load torque because of the above described phase lag. Because of this problem, some phase compensation means is neeeded.
As described below, however, it is difficult to attain this phase compensation. That is to say, the torque component of a rotary compressor includes harmonic components such as the second harmonic and the third harmonic in addition to the primary component of rotation corresponding to the running frequency of the compressor as shown in FIG. 20. The harmonic components have constant phase differences from each other. As indicated by the equation (1), the rotational speed W.sub.R is related to the excitation torque .DELTA.T.sub.r by integration. The rotational n-th component .DELTA.T.sub.n of the excitation torque will now be considered. By integrating the relation .DELTA.T.sub.n =.DELTA.T.sub.no sin n.omega.t, we obtain ##EQU4## Therefore, it is understood that the n-th component of the rotational speed is 2n/.pi. behind in phase the n-th component of the torque. The actual rotational speed of the rotor is equal to the result obtained by synthesizing all of these n-th components. Further, ratios of these components depend upon the running condition. Accordingly, the optimum phase compensation angle cannot be simply defined, resulting in a problem.